To improve the quality of workpieces welded, the following conventional method for joining workpieces has been known (Patent Literature 1). The method places two or more workpieces on a jig provided with a clamp unit, and joins the workpieces being clamped by the clamp unit for assembling a product. In this process, the method measures the amount of workpiece distortion occurring when the clamp unit clamps the workpieces, and on the basis of the measured distortion amount, calculates the workpiece correction amount required to leave no distortion after the joining of the workpieces together by welding. After the calculation, the clamp unit is moved, with the clamp position maintained, to deflect the workpieces by the correction amount at the clamp position before the welding.